Separation of the constituents of air



v c. c. VAN NUYS 2,423,273

SEPARATION OF THE CQNSTITUENTS OF AIR Filed Dec. 2, 1943 mma 1 H LH LHLMLHL L 7* t a w u 3' i E R3 Q L E *3 R3 INVENTOR. 6/4004 awn I 1 BY mmlgya A'I TORNEYS Fatenteol July 1, 1947 2,423,273 saraanrrou orruao'ous'rrrnnurs q or run Claude o.

Van Nuys, Greenwich, Conn, assignor to Air Reduction (Jompany,Incorporated, New

York, N; ill, a corporation of New York Application December 2, 1943,Serial No. 512,556

This invention relates to the separation of atmospheric air into itsconstituents by liquefaction and rectification and particularly toimprovemerits in the method and apparatus therefor,

in commercial liquefaction methods of separating oxygen from theatmosphere as heretofore practised, the necessary refrigeration toattain low temperatures is obtained by compression and subsequentexpansion of the air treated. This necessitates initial compression ofthe air to relatively high pressure of theorder of -20 atmospheres. Suchcompression can be attained only in reciprocating compressors whichrequire qil lubrication. The oxygen of the air under such pressures andat the temperatures resulting from compression causes decomposition ofthe lubri eating oil and the introduction to the air stream ofsubstantial proportions of hydrocarbons in addition to any which may bepresent. The use or special lubricating oils does not avoid this sourceof contamination.

Since the hydrocarbons are not removed from the air stream by the usualtreatment intended to separate moisture and carbon dioxide, they tend toaccumulate and eventually find their way to the compartment of theapparatus where liquid oxygen accumulates. The presence of such materialin contact with pure'oxygen, whether liquid or gaseous, is undesirable.Hence very elaborate attempts havebeen made heretofore to eliminate thehydrocarbons at some point intermediate compression ofthe air and theformation of liquid or vapor rich in oxygen in which the hydrocarbonsmight accumulate. Such attempts involve operations which interfere withthe efliciency of the cycle and the use of necessarily complicatedequipment. I Furthermore, these attempts have failed substantially toachieve the desired object. It is still possible for hydrocarbonsderived from the lubricating oil to accumulate at points in theapparatus where gases or liquids rich in oxygen are present.

It is the object of the present invention to provide a simple andeflective method andapparatus whereby the presence of hydrocarbons-resulting from decomposition of oil in the compression of the air canbe avoided and further improvements in the separationof oxygen from theatmosphere canbe obtained.

Other objects and advantages of the invention will be apparent as it isbetter understood by reference to the following specification and theaccompanying drawing, which illustrates diagrammatically the apparatussuitable for the practice of the invention.

1 Claim. (Cl. 62-1755) 2 In accordance with the present air to beseparated is not utilized to produce any invention, the

refrigeration in the cycle. .I employ instead a separate nitrogen cyclewhich acts as a heat transfer agent everywhere in the air cycle. The airneed be compressed in accordancewith my invention only to pressures notsubstantially higher than 3 atmospheres absolute. Usually a pressure of2 atmospheres or sufilcient pressure only to ensure movement of the. gasthrough the apparatus is needed to accomplish the purpose of theinvention. Such pressures are easily attainable in. variable types ofblowers which require no internal lubrication. Hence no lubricating oiland-decomposition products thereof are mingled with the air which iseventually separated to recover oxygen therefrom,

The nitrogen which afiords the refrigerative effect is compressed,circulated and expanded in such a way as to'provide sufficient cold sothat the air can be liquefied at low pressure. The nitrogen of therefrigeration cycle does not mingle at any point with the air undergoingseparation; Hence the presence of hydrocarbons in the nitrogen cycle isnot inimical to the method and affords no possibility of accumulation ofhydrocarbons in the presence of oxygen or products rich in oxygen. Thenitrogen can be compressed to the necessary pressure, for ex ample 2500pounds per square inch, in starting the apparatus and from 900 to 1000pounds per square inch thereafter, in any available form of compressor.Oil lubrication may be used. The quantityof nitrogen necessary for therefrigeration cycle will depend upon the size ofthe apparatus, but willnot in any case exceed the quantity of air contained in the airseparation cycle.

The procedure will be readily understood by reference to the drawing, inwhich 5 indicates a rectification column having the usual trays 6 andbubble caps 1. The column is provided at its tops with a diaphragm 8having an outlet 9 through which the efiluent; rich in nitrogen,escapes; Liquid oxygen accumulates in the bottom of the column, and thevapors therefrom rise through the trays 6 andbubble caps l in contactwith liquid introduced through a pipe H) which qflows downwardly overthe trays t. A; pipe H controlled by a valve 12 is connected tothebottom of the columnto permit purging in the event thateny impuritytends to accumulate in thepool of liquid oxygen.

Air. after initial compression to a pressure not greater than say 3atmospheres absolute, in any suitable apparatus but preferably in a,blower (not shown) is introduced through a pipe 13 to an exchanger Hwhere the air is cooled by heat exchange with the separatedconstituents. oxygen and nitrogen which are delivered to the exchangerthrough the pipes i 5 and I6 respectively. Afterflowing through tubes llof the exchanger, the oxygen and nitrogen are discharged through pipesl8 and I9 respectively. The air at low pressure and with its temperaturereduced. is delivered by a pipe to a liquefler 2| which is shown at thetop of the column 5, though it need not be disposed in this particularposition. The air is exposed in tubes 22 to the cooling effect of liquidnitrogen surrounding the tubes, and the resulting liquid air isdelivered through a pipe 23 and valve 24 to the pipe l0 and thus to thecolumn 5. As will be observed, all of the refrigerative effect isobtained through the liquid nitrogen surrounding the tubes 22. Neitherthe liquid nor the vapors resulting therefrom are mingled with the airundergoing liquefaction, and hence the latter is not contaminated by anyimpurities which may be present in the nitrogen.

The nitrogen travels in an independent refrigeration cycle. It isdelivered by a pipe 25 to.

a compressor 26 which may be a reciprocating compressor of the typeordinarily used in liquefaction plants. The nitrogen is compressed tothe required pressure for maintenance of the desired refrigerativeefiect. Usually in starting operation a pressure as high as 2500 poundsper square inch is necessary. Thereafter a pressure of 900 to1,000'pounds per square inch is ordinarily sufficient. Anypressure maybe used to afford sufficient refrigeration for maintenance oftheoperation. The compressed nitrogen is by a pipe- 29 to an exchanger30 where it circulates about tubes 3| carrying cold nitrogen returningin the cycle. The temperature of the compressed nitrogen is reduced, andit is delivered through a pipe 32 to a coil 33 immersed in the pool ofliquid oxygen at the bottom of the column 5. The nitrogen is thusfurther cooled in vaporizing the portion of the oxygen.

Thereafter the nitrogen is delivered through a pipe 34 to a liquefier 35and passes through tubes delivered by a pipe 21 to a cooler 28- andthence 36 therein in heat exchange relation with cold nitrogen vaporizedat a lower pressure. The liquid nitrogen is withdrawn through a'pipe 3'!and delivered through an expansion valve 38 and pipe 39 to the liquefier2| where the nitrogen liquid serves, as hereinbefore described, toeffect liquefaction of the low pressure air. A purge 40 controlled by avalve 4| permits removal of portions of the nitrogen liquid in the eventof any substantial contamination thereof, particularly through leakageof oxygen into the system.

The nitrogen vapor from the liquefier 2| is withdrawn through a pipe 42and delivered to the liquefier 35. It circulates about the tubes 35,effecting partial liquefaction of the high pressure nitrogen in thetubes. The nitrogen vapor is then delivered through a pipe 43 to theexchanger 30 and passes through the tubes 3| thereof to the pipe 25which again delivers the nitrogen to the compressor. Additional nitrogenmay be added from any source to the cycle as may. be necessary for, themaintenance thereof.

the air obviates that possibility. The oxygen is partially vaporized inthe bottom of the column 5 and the vapor may be withdrawn through a pipe44 controlled by a valve 45 and delivered to the pipe I5 and thence tothe exchanger i4. Alternatively, liquid oxygen may be withdrawn from thepool through a pipe 46 controlled by a valve 41 and delivered to aliquid oxygen pump 48 and thence by a pipe 49 to the pipe IS. Theoxygen, which is delivered eventually through the pipe i8, issubstantially free from contaminating impurities, particularly those ofa hydrocarbon origin.

The invention as described affords a practical and simple solution of aproblem which has existed for many years in the commercial production ofoxygen by liquefaction and rectification. It obviates most of the dangerinherent in the operation of methods of liquefaction in whichrefrigeration is maintained by initially compressing the air to elevatedpressures and the unavoidable contamination of the air with lubricatingOilS.

Various changes may be made in the details of procedure and in theapparatus as described without departing from the invention orsacrificing the advantages thereof.

I claim:

The method of separating the constituents of atmospheric air whichcomprises compressing the air to a relatively low pressure withoutcontaminating the air with lubricating oil, maintaining a separatenitrogen cycle including compression of the nitrogen to'relatively highpressure and liquefaction of the nitrogen by heat exchange with coldnitrogen vapors, liquefying the compressed air by heat exchange withliquid nitrogen, thereby vaporizing the nitrogen, and rectifying theliquid air to separate a liquid fraction consisting of oxygen.

CLAUDE C. VAN NUYS.

' REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

